14 October 2011

Tracking Cuvier’s Beaked Whales in the Pelagos Sanctuary

During the past summer, for the first time since Tethys carries out research in the Pelagos Sanctuary, a pod of six Cuvier’s beaked whales (Ziphius cavirostris) were eavesdropped and acoustically tracked for almost three hours. In the Mediterranean Sea Cuvier's beaked whales were often tracked using sophisticated acoustic tags and recorders, but only twice with horizontal surface towed arrays, by two different research teams. Despite several sightings occurring during the past years, this species was never tracked down by Tethys with a horizontal surface towed array - until this summer.

After almost six weeks I’m trying to piece together the memories about an incredible encounter with six Cuvier’s beaked whales peacefully swimming at the surface for more than three hours.
That day has been terribly hot and humid, with the temperature up to 38°C. The sea was mirror flat and the haze was flattering the summer colours and rendering all grey. The day was quite unfortunate with just a couple of sightings of striped dolphins in hours of navigation.
To find a bit of relief and coldness we just decided to jump into the water for a refreshing swim. Monica was the first one to get out of the water and to reach the sighting platform. With that flat sea she was craving to spot some pilot whales, a species that she had never seen before. And guess what? After not even one second on the observation point she started screaming “whale, whale, there is a whale!”. Of course, without even say “A” the people literally jumped out of the water. Everybody was ready for the "Big One".
Well, the "Big One" seemed to be disappeared. After more than 25 minutes it was not yet at the surface, something really unusual for a fin whale. But it was not a fin whale; it was a Cuvier’s beaked whale!
Cuvier’s beaked whales are some of the most "extreme" animals on the planet and one of the least known species of cetaceans. They are very difficult to detect visually at sea and to study, as they repeatedly perform long and deep dives, spending just a few minutes at the surface to rest. The characteristics of their sounds and their peculiar diving behaviour make it difficult even to follow them acoustically and to track their movements. Their acoustic abilities and features have been discovered only recently by using sophisticated audio recorders; Cuvier’s beaked whales do vocalize at great depth, at high frequency and with a narrow emission beam. Only little energy spreads toward the sea surface and this is why it is so difficult to even detect them. Horizontal towed arrays like the one that we use on the Pelagos R/V at Tethys, have never been really successful. This is particularly true for the Tethys cruises organized in the Ligurian Sea during the past 20 years; Cuvier’s beaked whales had never been eavesdropped and thus never tracked during field work.
So you can image our great surprise and excitement when the first beaked whale surfaced again with a second animal, then a third one arrived till they became six. A pod of six beaked whales was swimming close to the boat under the amazed and unbelieving look of the crew. These animals spent up to 5 minutes at the surface after performing short dives of about 25 minutes of duration. And it was actually during one of these short dives that Roberto, the skipper, and myself, while seating at the PC, noticed high frequencies click trains on the real-time sound spectrogram. We could not believe our eyes; could that be the clicks from the beaked whales or it was something else? The only way to find it out was to try to track the clicks; the risk to lose track of the animals was high but we were too excited about the idea of the clicks coming from THOSE animals. So armed with patience and hoping in some good luck we started following the clicks on the spectrogram and on the tracking software. And, punctually, a few seconds after the clicks disappeared from the PC screen, we could hear people screaming outside, indicating and pointing at the whales.
We made it! We just tracked Cuvier’s beaked whales for the first time in the history of the Tethys’ Cetacean Sanctuary Research Project. After this first attempt we repeated the tracking a few other times, just to be sure it was not only a matter of luck. And again the whales were there, just a few meters apart from the boat. So we actually made it!
After more than three hours we decided to let the beaked whales go, but just because we spotted some fin whales’ blows not far from us. But this is another story.
Nino Pierantonio

Recommended readings:
Frantzis A., Goold J.C., Skarsoulis E.K., Taroudakis M.I., Kandia V., 2002. Clicks from Cuvier’s beaked whales, Ziphius cavirostris (L). J. Acoust. Soc. Am., Vol. 112, No. 1: 34-37.

Johnson M., Madsen P.T., Zimmer W.M.X., Aguilar de Soto N., Tyack P. 2004. Beaked whales echolocate on prey. Proc. R. Soc. Lond. B (Suppl.) 271: S383–S386.

Zimmer W.M.X., Johnson M., Madsen P.T., Tyack P. 2005. Echolocation clicks of free-ranging Cuvier’s beaked whales (Ziphius cavirostris). J. Acoust. Soc. Am. 117 (6): 3919-3927.

No comments: